In recent years, attendant on the spread of networks, documents hitherto distribution in the form of printed matter have come to be transmitted in the form of electronic documents. Further, books and magazines have come to be often provided in the form of the so-called electronic publishing. In order to read these pieces of information, reading from CRTs (cathode ray tubes) and liquid crystal displays of computers has conventionally been widely conducted.
However, in a light emission type display such as the CRT, it has been pointed out that the display causes conspicuous wearing on an ergonomic ground and, therefore, is unsuited to long-time reading. In addition, even a light reception type display such as a liquid crystal display is said to be similarly unsuited to reading, because of the flickering which is intrinsic of fluorescent tubes. Furthermore, both of the types have the problem that the reading place is limited to the places where computers are disposed.
In recent years, reflection type liquid crystal displays not using a backlight have put to practical use. However, the reflectance in non-display (display of white color) of the liquid crystal is 30 to 40%, which means a considerably bad visibility, as compared with the reflectance of printed matter printed on papers (the reflectance of OA papers and pocket books is 75%, and the reflectance of newspapers is 52%). In addition, the glaring due to reflectors and the like are liable to cause wearing, which also is unsuited to long-time reading.
In order to solve these problems, the so-called paper-like displays and the so-called electronic papers have been being developed. The media mainly utilize coloration by moving colored particles between electrodes through electrophoresis or by rotating dichroic particles in an electric field. In these methods, however, the gaps between the particles absorb light, with the result that contrast is worsened and that a practical-use writing speed (within one sec) cannot be obtained unless the driving voltage is 100 V or higher.
As compared with the displays of these systems, electrochemical display devices for color development based on an electrochemical action (electrochromic display: ECD) is better in contrast, and they have already been put to practical use as light control glass or timepiece displays. It should be noted here that the light control glass and timepiece displays are not directly suited to the electronic paper or the like uses, since it is intrinsically unnecessary to perform matrix driving. Besides, they are generally poor in quality of black color, and the reflectance thereof remains at a low level.
In addition, in such displays as electronic papers, they are continuedly exposed to solar light or room light on a use basis, and, in an electrochemical display device put to practical use in the light control glass and timepiece displays, an organic material is used for forming black-colored portion, which leads to a problem concerning light resistance. In general, organic materials are poor in light resistance, and the black color concentration thereof is lowered through fading when used for a long time.
In order to solve these technical problems, there has been proposed an electrochemical display device using metal ions as a material for color change. In the electrochemical display device, the metal ions are preliminarily mixed into a polymer electrolyte layer, the metal is deposited and dissolved by electrochemical redox reactions, and the change in color attendant on this is utilized to perform display. Here, for example, when the polymer electrolyte layer contains a coloring material, it is possible to enhance the contrast in the case where the color change occurs.
Meanwhile, in the metal deposition type electrochemical display device based on deposition and dissolution of the metal, a threshold voltage which is the deposition overvoltage is utilized to achieve display. In each pixel, the metal is deposited when a minus voltage in excess of the threshold voltage is impressed between electrodes arranged in a matrix form, whereas the metal is dissolved when a plus voltage is impressed between the electrodes.
When it is tried to drive a display apparatus based on a simple matrix system by utilizing the threshold voltage, the degree of coloration of the pixel would vary according to the sequence of selection.
For example, where display as shown in FIG. 19 is conducted by use of row electrodes and column electrodes which are arranged in a matrix, the impression of the voltage on the electrodes is performed following a time sequence as shown in FIG. 20A. Specifically, while a scan pulse voltage is impressed sequentially on the row electrodes, a data pulse voltage is impressed on the column electrodes only at the time of coloration. As a result, a voltage (scan pulse voltage+data pulse voltage) in excess of the threshold voltage is impressed on selected pixels to cause coloration (deposition of metal), and only the scan pulse voltage is impressed on non-selected pixels, so that metal deposition does not occur in the non-selected pixels, and non-colored state is maintained there.
In this case, the driving is conducted on a line sequence basis, and there has been confirmed a phenomenon in which the degree of coloration in the pixels arranged on a common column electrode becomes gradually deeper according to the sequence of selection. This is due to the following. In the pixel in which the coloration (deposition of silver) has once occurred, a tiny current flows and the coloration proceeds even where a voltage below the threshold voltage is impressed. Therefore, as is clear from an example of current waveform in FIG. 20B, the scan pulse voltage is impressed on the previously colored pixel, and a current flows there, also at the time of coloration in the subsequent pixel. For example, in a pixel (R1, C3), after the selection period for writing by impressing the scan pulse voltage at a row electrode R1, the data pulse voltage is impressed on a column electrode C3 also during the selection periods of the row electrodes R2 and R3. Similarly, in a pixel (R2, C3), after the selection period for writing by impressing the scan pulse voltage is impressed on the row electrode R2, the data pulse voltage for the column electrode C3 is impressed during the selection period of the row electrode R3. When the threshold voltage is once exceeded, the impression of only these data pulse voltages causes a current to flow and causes the coloration (deposition of metal) to proceed.
Thus, the pixel selected in the beginning stage of image rewriting undergoes writing repeatedly, dependent on the data for the following pixels. As a result, the substantial writing time is elongated according to the sequence of scanning, so that the writing concentration would be enhanced more than required.
In addition, in the conventional driving method as above, the metal such as silver must be deposited stably at the time of coloration, so that the voltage must be impressed over a certain period. Where the silver deposition characteristic of the panel is not uniform, the addressing time must be conditioned to accord to the pixel which is the worst in the characteristic. Therefore, the addressing period is elongated, and the image rewriting time would be increased.
The present invention has been proposed for the purpose of solving these problems. Accordingly, it is an object of the present invention to provide a method of driving a display apparatus with which it is possible to moderate the non-evenness of pixels formed, and to shorten the time for rewriting the image display.